FIG. 5 illustrates a single element photodetector 60 in accordance with yet another alternative preferred embodiment of the present invention. Photodetector 60 includes a single active pixel 60a and a wedge-shaped substrate 62. Again, the substrate 62 has a first surface 62a and a second surface 62b. The substrate 62 is wedge-shaped such that surface 62b extends at an angle represented by arrow 64, preferably at least about 5 degrees, and more preferably about 10 degrees. The incident radiation 18 impinging the single detector element 60a is thus reflected from surface 62b away from detector element 60a, thus preventing the channel spectrum effect.
Interferometric spectrometers are now used for hyper-spectral and ultra-spectral imaging of scenes. These scenes may be relatively close to the spectrometer, as in biomedical applications. Alternatively, they may be extremely distant from the spectrometer, as with airborne and spaceborne instruments obtaining data from the Earth and extraterrestrial objects.
There are several types of interferometric spectrometers, including Fourier transform interferometers and tuneable Fabry-Perot Etalons. Interferometric spectrometers are subject to an undesirable artifact referred to as xe2x80x9cchannelingxe2x80x9d or xe2x80x9cchannel spectrum effectxe2x80x9d (see R. Beer, Remote Sensing by Fourier Transform Spectrometry, John Wiley and Sons; 1992, and R. J. Bell, Introductory Fourier Transform Spectroscopy, Academic Press, 1972). This effect exists in both imaging and non-imaging instruments. The effect produces instrumental artifacts in resulting data output by a detector. Usually these artifacts are such that they cannot be compensated for by careful instrument calibration. Because of this, those sections of data that are corrupted by this artifact are deleted, resulting in a loss of data and system performance degradation.
The channel spectrum effect is attributable to reflections between parallel surfaces within the instrument. In many cases, these parallel surfaces may be eliminated by fairly simple and well known means. Transparent slabs occur as elements of an etalon, or in the beam splitter in a Michelson interferometer. These are typically made wedge-shaped to eliminate the plane parallel surfaces. The detectors used to sense the radiation may be of single element construction or they may consist of an array of elements. Their composition and structure are dependent on the wavelength of the radiation to be sensed. For example, in the infrared regime comprising the wavelength range 1 um to 30 um, there are available individual detectors operating in either the photovoltaic or photoconductive mode, and there are also arrays of elements usually operating in the photovoltaic mode. In an array, each detector element is referred to as a pixel. The detectors typically consist of a thin photoactive layer or layers supported by a substrate. These devices are currently configured as flat slabs with plane parallel faces. Because of the detailed properties of the devices, a portion of the incident radiation is reflected by the surfaces in such a way as to generate a channel spectrum.
It is therefore a principal object of the present invention to provide an infrared detector array for eliminating the channel spectrum effect.
It is still another object of the present invention to provide a photodetector array that eliminates the channel spectrum effect without the need for complex and expensive electronic signal processing equipment.
It is still another object of the present invention to provide a photodetector array that eliminates the channel spectrum effect, such that only minor modifications are required of the detector array in its construction.
The above and other objects are provided by a photodetector in accordance with preferred embodiments of the present invention. In one preferred embodiment, the photodetector comprises a photoactive layer that is supported on a substrate. The photoactive layer incorporates one or more pixels for receiving incident radiation. The substrate incorporates first and second surfaces, with the photoactive layer being supported on the first surface. The second surface is formed non-parallel to the first surface. More specifically, the entire substrate comprises a xe2x80x9cwedgexe2x80x9d shaped form. The angle of divergence of the second surface of the substrate from the first surface may vary, but is preferably at least about five degrees, and more preferably about ten degrees. The optimum angle is dependent on detector element size, substrate material and thickness.
In the preferred embodiments, incident radiation passing through the pixel or pixels is reflected at the second surface of the substrate. The incident radiation is reflected such that it does not return towards the pixel through which it has passed, but rather is reflected away from the pixel through which it has just passed. In the first embodiment, the incident radiation passes through the pixel and is reflected by the second surface of the substrate away from the pixel. In a second embodiment, the incident radiation first passes through the second surface of the substrate before impinging the pixel (or pixels), and is reflected back towards the second surface of the substrate. The second surface is formed non-parallel to the first surface such that the incident radiation is further reflected by the second surface away from the pixel from which it was reflected.
The preferred embodiments allow the undesirable channel spectrum effect to be eliminated without the need for complicated electronic signal processing equipment or costly modifications to photodetector structures. The photodetector of the present invention further can be manufactured without significantly increasing the complexity of the manufacturing process or adding appreciably to the overall size of the detector structure.